Glaucoma is a potentially blinding disease, distinguished by elevated intraocular pressure (IOP), which if left untreated can lead to optic nerve damage resulting in blindness. Today's glaucoma therapy consists of mainly monitoring, and lowering the intraocular pressure by medical or surgical therapy.
Measurement of intraocular pressure in glaucoma patients is usually performed in a doctor's office, using one of the presently available external tonometers. Clinical measurement of intraocular pressure is performed by deforming the globe of the eye and correlating the force responsible for the deformation to the pressure within the eye. Both indentation and applanation tonometers deform the globe of the eye while measuring intraocular pressure. A third type of tonometer, the non-contact tonometer, measures the time required to deform the corneal surface in response to the force produced by a jet of air. The accuracy of the non-contact tonometer is diminished with higher intraocular pressures and in eyes with abnormal corneas or poor fixation.
Most tonometers require the application of a topical anesthetic following which the tonometer is applied to the corneal surface, by or under the supervision of a physician. The unequivocal need to have a highly trained professional available during intraocular measurements, in addition to the risk of corneal abrasion, reactions to topical anesthetics, and transmission of infectious agents limit the accessibility and ease of monitoring intraocular pressure in glaucoma patients.
The intraocular pressure in normal people varies throughout the day. Abnormal pressure peaks may occur at odd hours, e.g. very early in the morning, or at times when it is inconvenient to see the patient in the doctor's office and impractical to record the intraocular pressures. This fluctuation is often accentuated in people with glaucoma.
Knowledge of variations in intraocular pressure is important for the diagnosis, treatment, and eventually prognosis of glaucoma. An intraocular pressure measurement at one point in time may not tell the whole story. In patients for whom elevated intraocular pressures can not be documented during visits into the doctor's office, diurnal curves are considered to be a great value in the diagnosis and treatment of glaucoma, and to evaluate the response to glaucoma therapy during subsequent visits. The diurnal intraocular pressure curves can provide information on both peak intraocular pressure, and the range of diurnal pressure variations. Documentation of diurnal intraocular pressure variations is crucial in the study and assessment of dose response studies of anti-glaucoma medications. The need to verify and document diurnal pressure variations is especially important in patients with seemingly controlled intraocular pressures, but with progressive glaucomatous damage.
Assessing diurnal variations of intraocular pressure requires repeated measurements around the clock. Methods used include inpatient measurements, office measurements, and outpatient-hospital combinations. The major disadvantages of these latter procedures are their cost, the drastic modification introduced of the patient's normal activities, and possible introduction of exogenous factors that affect the diurnal pressure, such as in changing the normal sleep pattern, and hence possibly falsely varying the measured intraocular pressures. Another major disadvantage is the gradual reduction of the intraocular pressure induced by multiple manipulations and pressure applications on the corneal surface, which result in an iatrogenic reduction in the intraocular pressure, a phenomenon known as the “Tonography effect”.
Attempts have been made to have patients or their relatives measure the intraocular pressure at home during various times of the day either to look for elevated intraocular pressures or to assess the quality of intraocular pressure control. This could be a source of corneal abrasions and infections, in addition to possibly initiating topical anesthetic abuse. Moreover, the results and accuracy of home tonometry have been highly variable.
U.S. Pat. No. 5,833,603 to Kovacs' et al. issued Nov. 10, 1998 disclosed a biosensing transponder for implantation in an organism, which includes a biosensor and a transponder. Although one embodiment describes a biosensing transponder with an implantable inductive pressure sensor to allow remote sensing and retrieval of static and dynamic pressure information, no details of the construction of the inductive sensor are provided.
An article entitled “Miniature Passive Pressure Transensor for Implanting in the Eye” by Carter C. Collins, issued by IEEE on Bio-Medical Engineering in April 1967, disclosed an intraocular pressure sensor including a pair of parallel, coaxial, flat spiral coils, which constitutes a distributed resonant circuit whose frequency varies with relative coil spacing. However, the spiral coils of the intraocular pressure sensor of Collins are produced by hand winding and hand assembly, which is both costly and inefficient.
Another article entitled “A System for Passive Implantable Pressure Sensors” by Rosengren et al., issued by Sensors and Actuators A in 1994, disclosed an implantable sensor, which is a capacitive micromachined silicon structure, together with a coil, constitutes a passive radio-frequency resonator. The coil is made up of 50 μm diameter gold wire, wound on a plastic fixture with a diameter of 5 mm. The capacitor is glued to the fixture, and the coil ends are bonded to the top and bottom surface of the capacitor. Unfortunately, this sensor has a large size of 5 mm diameter and 2 mm thickness. In addition, the device also uses hand wound coils and assembly by hand.
U.S. Pat. No. 4,127,110 to Bullara issued Nov. 28, 1978 discloses a wireless, surgically implantable pressure transducer for measuring pressure of fluid or tissue in a body chamber such as a brain ventricle of a patient suffering hydrocephalus or after head injury. The transducer includes a helical inductor coil and a capacitor connected in parallel to form a resonant L-C circuit. One of these reactive components is variable, and a bellows is mechanically connected to the variable component to vary the value of capacitance or inductance and hence the resonant frequency of the L-C circuit in response to pressure changes of fluid in which the bellows is immersed. The resonant frequency of L-C circuit is detected and measured by an external source of variable-frequency energy such as a grid-dip oscillator or a solid state equivalent. Unfortunately, the helical inductor coil needs hand winding of gold wire around a core having an outside diameter of 0.25 inch, thus the transducer has a large size.
U.S Pat. No. 4,026,276 to Chubbuck issued May 31, 1977 discloses a pressure monitoring apparatus implantable in the cranium to measure intracranial pressure. The apparatus comprises a passive resonant circuit having a natural frequency influenced by ambient pressure. The resonant circuit has inductance and capacitance capability for comparing the local environmental pressure to that of a volume of gas trapped inside the apparatus. The environmental pressure is measured by observation of the frequency at which energy is absorbed from an imposed magnetic field located externally of the cranium. However, this apparatus has a cylindrical inductance coil, which needs hand winding and hand assembly.
U.S. Pat. No. 4,628,938 to Lee issued Dec. 16, 1986 and U.S. Pat. No. 5,830,139 to Abreu issued Nov. 3, 1998 disclose non-invasive, continuous applanation tonometers including pressure sensors for measuring intraocular pressure, which is performed by deforming the globe and correlating the force responsible for the deformation to the pressure within the eye. Unfortunately, these techniques require a highly trained professional available during intraocular pressure measurements, in addition to the risk of corneal abrasion, reactions to topical anesthetics, and transmission of infectious agents.
There is a need, therefore, for an implantable intraocular pressure measuring microdevice that overcomes the above difficulties.